This invention relates to an information recording method of recording information by use of an information recording medium capable of recording information by the radiation of an energy beam, and more particularly to an information recording method suitable for overwrite for recording new information while erasing existing information in accordance with an arbitrary modulation method by use of a recording film capable of high velocity crystallization.
A method of recording information which contracts a luminous flux such as a laser beam to a fine beam spot, records and reproduces signals in a high density on a rewritable optical recording medium and records new information while erasing the existing information that has once been recorded is disclosed, for example, in Japanese Patent Laid-Open No. 259229/1987. If a recording film capable of high velocity erasure is used as described in the above-identified reference, it makes possible to effect crystallization in substantially the same time as the radiation time of the laser spot for recording by rendering the recording medium amorphous. The two levels of power, a high power level and an intermediate level, are used to write, both levels being higher than the power level required for reading. This method provides the advantage in that new information can be recorded while existing information is being erased (overwrite).
When recording and erasure are made onto a phase change type optical recording film by the conventional technique described above, the recording film has the composition satisfying the relation EQU .alpha.&lt;.tau.&lt;.beta. (1)
where .tau. is the time necessary for the recording film to exhibit the phase change, from the amorphous state to the crystalline state; or from one of the crystalline states to the other; or a crystallization time; or a phase change time. .alpha. is the time in which the temperature of the recording medium passes through the crystallization or the phase change temperature zone at the fall portion of a given laser beam pulse from the high power level. Finally .beta. is the time in which the temperature of the recording medium passes through the crystallization or phase change temperature zone after the passage of the optical spot at the continuous portion of the high power level of the given laser beam pulse. When the recording film satisfying the relation (1) above is used, a reproduction waveform can be obtained whose voltage changes drastically only near the rise portion of the recording pulse from the intermediate power level to the high power level and near the fall portion from the high power level to the intermediate power level. At this time, the crystalline area occupies the major proportion at the rise portion while the amorphous area occupies the major proportion at the fall portion. Furthermore, the crystalline area and the amorphous area are copresent (intermediate state in the portion to which the beam of the intermediate power level is radiated and in the portion between the rise portion and the fall portion to which the beam of the high power level is radiated continuously).